Image sensing devices are known in the art. For many applications, an image sensing device is formed in an integrated circuit “chip” and the so-formed chip is included as part of an imaging system. As one example, image sensing devices are included in digital cameras. The image sensing devices typically record color images by employing pixel sensors to independently sense the image to be recorded in red, green and blue color channels. The channel signals generated by the pixel sensors are then stored and subsequently processed to generate a corresponding image.
The spectral response of such image sensing devices varies from chip to chip, primarily due to manufacturing variations. In general, a transformation is applied to convert the generated channel signals into a standard color space for processing by the system of which the image sensing device is a part. Such a transformation is known as a color transformation. Each imager is calibrated by determining its spectral response and computing a color transformation based on the determined spectral response. The computed calibration parameters are used as input to the color transformation.
The spectral response of an image sensing device is primarily a function of the characteristics of the pixel sensors collectively (including chip sensitivities and filters applied to the image sensing device) and any additional filters (“prefilters”) that are used to shape the overall response (for example, by rejecting infrared light). The spectral characteristics of the prefilters also vary due to manufacturing variations. Furthermore, the image sensing device may be packaged as part of an imaging sub-assembly, that includes the image sensing device along with prefilter(s) and/or other elements. The calibration parameters for the entire imaging sub-assembly are typically computed before incorporating the sub-assembly into an imaging system such as an assembled camera.
To compute the calibration parameters, both specialized test setups and knowledge of color theory are typically employed, neither of which is typically available to an electronics assembler. Therefore, it is desirable that the calibration parameters be computed before providing the imaging sub-assembly to the imaging system manufacturer, and that the computed calibration parameters be provided to the imaging system manufacturer along with the imaging sub-assembly in a manner that is not prone to mismatches between a particular imaging sub-assembly and the computed calibration parameters for that sub-assembly.